Development of thermo-mechanical processing for fabricating highly durable β-type Ti-Nb-Ta-Zr rod for use in spinal fixation devices

The mechanical strength of a beta titanium alloy such as Ti-Nb-Ta-Zr alloy (TNTZ) can be improved significantly by thermo-mechanical treatment. In this study, TNTZ was subjected to solution treatment, cold caliber rolling, and cold swaging before aging treatment to form a rod for spinal fixation. The {110}(beta) are aligned parallel to the cross-section with two strong peaks approximately 180 degrees apart, facing one another, in the TNTZ rods subjected to cold caliber rolling and six strong peaks at approximately 60 degrees intervals, facing one another, in the TNTZ rods subjected to cold swaging. Therefore, the TNTZ rods subjected to cold swaging have a more uniform structure than those subjected to cold caliber rolling. The orientation relationship between the alpha and beta phases is different. A |110|(beta)/|121|(alpha), (112)beta//(210)alpha orientation relationship is observed in the TNTZ rods subjected to aging treatment at 723 K after solution treatment and cold caliber rolling. On the other hand, a |110|(beta)//|001|(alpha), (112)(beta)//(200)(alpha) orientation relationship is observed in TNTZ rod subjected to aging treatment at 723 K after cold swaging. A high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa indicate that aging treatment at 723 K after cold swaging is the optimal thermo-mechanical process for a TNTZ rod. (C) 2012 Elsevier Ltd. All rights reserved.